transport layer (tcp/udp) - courses.cs.washington.edu · transport layer protocols •provide...

Transport Layer (TCP/UDP)

Recall the protocol stack

Computer Networks 2

Application – Programs that use network service

Transport – Provides end-to-end data delivery

Network – Send packets over multiple networks

Link – Send frames over one or more links

Physical – Send bits using signals

Transport layer

Provides end-to-end connectivity to applications

CSE 461 University of Washington 3

Host Host

TransportNetwork

Client

TransportNetwork

Server

Transport layer protocols

•Provide different kinds of data delivery across the network to applications


Unreliable ReliableMessages Datagrams (UDP)Bytestream Streams (TCP)

Comparison of Internet transports

•TCP is full-featured, UDP is a glorified packet


TCP (Streams) UDP (Datagrams)Connections Datagrams

Bytes are delivered once, reliably, and in order

Messages may be lost, reordered, duplicated

Arbitrary length content Limited message sizeFlow control matches

sender to receiverCan send regardless

of receiver stateCongestion control matches

sender to networkCan send regardless

of network state

Socket API

•Simple abstraction to use the network• The “network” API (really Transport service) used to write

all Internet apps• Part of all major OSes and languages; originally Berkeley

(Unix) ~1983•Supports both Internet transport services (Streams

and Datagrams)


Socket API (2)

• Sockets let apps attach to the local network at different ports


Socket,Port #1

Socket,Port #2

Socket API (3)•Same API used for Streams and Datagrams


Primitive MeaningSOCKET Create a new communication endpointBIND Associate a local address (port) with a socketLISTEN Announce willingness to accept connectionsACCEPT Passively establish an incoming connectionCONNECT Actively attempt to establish a connectionSEND(TO) Send some data over the socketRECEIVE(FROM) Receive some data over the socketCLOSE Release the socket

Only needed for Streams

To/From for Datagrams

Ports

•Application process is identified by the tuple IP address, transport protocol, and port• Ports are 16-bit integers representing local “mailboxes”

that a process leases•Servers often bind to “well-known ports”• <1024, require administrative privileges

•Clients often assigned “ephemeral” ports• Chosen by OS, used temporarily


Some Well-Known Ports


Port Protocol UseTCP/20, 21 FTP File transfer

TCP/22 SSH Remote login, replacement for TelnetTCP/25 SMTP EmailTCP/80 HTTP World Wide Web

TCP/443 HTTPS Secure Web (HTTP over SSL/TLS)TCP/3306 MYSQL MYSQL database access

UDP/53 DNS Domain name service

Full list: https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.txt

https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.txt

Topics

• Service models• Socket API and ports• Datagrams, Streams

• User Datagram Protocol (UDP)• Connections (TCP)• Sliding Window (TCP)• Flow control (TCP)• Retransmission timers (TCP)• Congestion control (TCP)


User Datagram Protocol (UDP)

•Used by apps that don’t want reliability or bytestreams• Like what?


User Datagram Protocol (UDP)

•Used by apps that don’t want reliability or bytestreams• Voice-over-IP • DNS• DHCP• Games

(If application wants reliability and messages then it has work to do!)


Datagram Sockets


Client (host 1) Server (host 2)Time

request

reply

Datagram Sockets (2)


Client (host 1) Server (host 2)Time

1: socket 2: bind1: socket

6: sendto

3: recvfrom*4: sendto

5: recvfrom*

7: close 7: close*= call blocks

request

reply

UDP Buffering


App

Port Mux/Demux

App AppApplication

Transport(UDP)

Network (IP) packet

Message queues

Ports

UDP Header

•Uses ports to identify sending and receiving application processes•Datagram length up to 64K•Checksum (16 bits) for reliability


UDP Header (2)

•Optional checksum covers UDP segment and IP pseudoheader• Checks key IP fields (addresses)• Value of zero means “no checksum”


TCP

•TCP Consists of 3 primary phases:• Connection Establishment (Setup)• Sliding Windows/Flow Control• Connection Release (Teardown)

Connection Establishment

•Both sender and receiver must be ready before we start the transfer of data• Need to agree on a set of parameters• e.g., the Maximum Segment Size (MSS)

•This is signaling• It sets up state at the endpoints• Like “dialing” for a telephone call



Three-Way Handshake• Used in TCP; opens connection for

data in both directions• Each side probes the other with a

fresh Initial Sequence Number (ISN)• Sends on a SYNchronize segment• Echo on an ACKnowledge segment

• Chosen to be robust even against delayed duplicates

Active party(client)

Passive party(server)


Three-Way Handshake (2)

•Three steps:• Client sends SYN(x)• Server replies with SYN(y)ACK(x+1)• Client replies with ACK(y+1)• SYNs are retransmitted if lost

•Sequence and ack numbers carried on further segments

1

2

3



SYN (SEQ=x)

SYN (SEQ=y, ACK=x+1)

(SEQ=x+1, ACK=y+1)Time



•Suppose delayed, duplicate copies of the SYN and ACK arrive at the server!• Improbable, but anyhow …



SYN (SEQ=x)

(SEQ=x+1,ACK=z+1)



•Suppose delayed, duplicate copies of the SYN and ACK arrive at the server!• Improbable, but anyhow …

•Connection will be cleanly rejected on both sides J



SYN (SEQ=x)


(SEQ=x+1,ACK=z+1)

XXREJECT

REJECT

TCP Connection State Machine

•Captures the states ([]) and transitions (->)• A/B means event A triggers the transition, with action B

Both parties run instances of this state

machine

TCP Connections (2)

• Follow the path of the client:

TCP Connections (3)

• And the path of the server:

TCP Connections (4)

• Again, with states …


LISTEN

SYN_RCVD

SYN_SENT

ESTABLISHED

ESTABLISHED

1

2

3

Active party (client) Passive party (server)

SYN (SEQ=x)


(SEQ=x+1, ACK=y+1)Time

CLOSEDCLOSED

TCP Connections (5)

•Finite state machines are a useful tool to specify and check the handling of all cases that may occur

•TCP allows for simultaneous open• i.e., both sides open instead of the client-server pattern• Try at home to confirm it works J


transport layer (tcp/udp) - courses.cs.washington.edu · transport layer protocols •provide...

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